This invention relates to a semiconductor device for use in a field effect transistor and, more particularly, to the semiconductor device having a quantum wire structure.
It is known in the art that a semiconductor device comprises a quantum wire structure which includes a heterojunction structure formed between a first semiconductor layer and a second semiconductor layer. Such a semiconductor device may be used as a field effect transistor which may be called a high electron mobility transistor.
A conventional semiconductor device of the above-mentioned type is disclosed in a title of "One-Dimensional Subbands and Mobility Modulation in GaAs/AlGaAs Quantum Wires" contributed by K. Ismail et al to Applied Physics Letters, Vol. 54, March 1989, pages 1130 to 1132.
In the conventional semiconductor device disclosed by K. Ismail et al, an undoped GaAs layer is used as the first semiconductor layer while an n-type AlGaAs layer is used as the second semiconductor layer.
Herein, it is to be noted that the undoped GaAs layer has a first conduction band consisting of three valleys which may be called a first .GAMMA.-valley, a first L-valley, and a first X-valley.
Similarly, the n-type AlGaAs layer has a second conduction band consisting of three valleys which may be called a second .GAMMA.-valley, a second L-valley, and a second X-valley.
Now, it will be assumed that the first .GAMMA.-valley, the first L-valley, and the first X-valley have a first .GAMMA.-valley energy level, a first L-valley energy level, and a first X-valley energy level, respectively. In addition, it will be assumed that the second .GAMMA.-valley, the second L-valley, and the second X-valley have a second .GAMMA.-valley energy level, a second L-valley energy level, and a second X-valley energy level, respectively.
The first .GAMMA.-valley energy level is defined by an energy level of the bottom of the first .GAMMA.-valley. The first L-valley energy level is defined by an energy level of the bottom of the first L-valley. The first X-valley energy level is defined by an energy level of the bottom of the first X-valley. The second .GAMMA.-valley energy level is defined by an energy level of the bottom of the second .GAMMA.-valley. Similarly, the second .GAMMA.-valley energy level is defined by an energy level of the bottom of the second .GAMMA.-valley. The second L-valley energy level is defined by an energy level of the bottom of the second L-valley. The second X-valley energy level is defined by an energy level of the bottom of the second X-valley.
In the conventional semiconductor device, the first .GAMMA.-valley has at least one one-dimensional subband inasmuch as the first .GAMMA.-valley energy level is lower than each of the second .GAMMA.-valley, the second L-valley, and the second X-valley energy levels so that the first .GAMMA.-valley has a quantum barrier in a heterojunction surface. On the other hand, the one-dimensional subband is not formed in each of the first L-valley and the first X-valley inasmuch as each of the first L-valley and the first X-valley energy levels is greater than at least one of the second .GAMMA.-valley, the second L-valley, and the second X-valley energy levels. As a result, electrons behave similar to electrons in a bulk semiconductor device.
On applying a high electric field on the conventional semiconductor device, electrons in the first .GAMMA.-valley are transited from the first .GAMMA.-valley to the first L-valley. Inasmuch as the effective mass of electrons is greater in the first L-valley than that of electrons in the first .GAMMA.-valley, the average drift velocity of electrons decreases till a drift velocity defined by materials of the conventional semiconductor device when the one-dimensional subband is not formed in the first L-valley as described above. As a result, it is difficult to improve an electron transport property at the high electric field in the conventional semiconductor device.